This invention relates to a process and apparatus for purification of phosphate-containing sewage with biological phosphorus elimination, in which the settleable and/or nonsettleable sewage components are treated with activated sludge in successive treatment tanks, first under anaerobic and then under aerobic conditions, and in which return activated sludge is passed from the secondary settling tank into the stage operating under anaerobic conditions.
Unlike nitrogen compounds, phosphorus compounds can be removed from sewage only if they can be separated from the sewage in the solid state on the basis of chemical or biochemical reactions. The previously required conversion of phosphates from the water to the solid state in this case can take place only by incorporation into the biomass or by chemical precipitation.
For many years chemical precipitation processes have been used in a multiplicity of settling plants for elimination of phosphorus. Processes, in which the phosphorus elimination takes place in a biological or biological-chemical manner, represent a comparatively recent development. Thus, in the purely biological process an effort is made to fix biologically in excess sludge the entire phosphorus that is to be eliminated. On the other hand, in the biological-chemical process the biological effect is combined in a special technique with an exceptionally economically operating lime precipitation.
In the biological process, use is made of the phenomenon that the phosphorus contained in the sewage is released or bound by certain microorganisms depending on the conditions under which sewage and activated sludge are in contact. In this case the phosphorus is present in the sewage practically exclusively as phosphate, namely mostly as dissolved phosphate and in a considerably smaller amount as bound, particulate phosphate. If in a biological settling plant operating according to the activated sludge process, the activated sludge is subjected to a constant change from anaerobic conditions (absence of dissolved oxygen, nitrite and nitrate) and aerobic conditions (presence of dissolved oxygen), it becomes apparent that the activated sludge under anaerobic conditions releases phosphate and the activated sludge under aerobic conditions absorbs phosphate. Thus there occurs the phosphate release in the anaerobic phase in contrast to a phosphate absorption in the aerobic phase. In this connection, it has been shown in all the studies conducted in this field that the degree of phosphate absorption under aerobic conditions is directly related to the degree of previous phosphate redissolution in the anaerobic phase.
The fact that the extent of the phosphate absorption is always higher than the extent of previous redissolution is essential for a biological phosphorus elimination. Since the separation of purified sewage from the biological sludge takes place following the aerobic phase, a net elimination remains, i.e., a lowered phosphorus concentration in the discharge and an increased phosphorus concentration in the excess sludge of the settling plant.
Therefore to increase the elimination performance an effort is made to produce as high as possible an amount of excess sludge and/or to permit the phosphorus content in the excess sludge to rise to as high a value as possible.
The specific excess sludge production is thus determined by the sludge loading.
The phosphorus content of the excess sludge can be increased on the basis of the above-mentioned relationships, namely by an intensification of the phosphorus redissolution and the subsequent intensified phosphorus absorption resulting from it.
If it is desired to influence the phosphorus elimination by changing the phosphorus redissolution/phosphorus absorption in a biological settling plant, as other conditions there must be further considered:
1. The presence of nitrite and nitrate prevents phosphorus redissolution. Thus, no nitrite- or nitrate-containing partial streams should flow into the anaerobic part of the plant.
2. The availability of an easily degradable substrate accelerates the phosphorus redissolution.
In the processes developed so far for biological phosphorus elimination, these two factors have been taken into account in different ways. In the simplest process so far, the so-called A/O process, which, for example, is described in the study of Krichten, D. J., Hong, S. N., Tracy, K. D., Applied Biological Phosphorus Removal by the A/O Process, Internat. Conf. "Phosphorus in the Environment," Lisbon, July 1985, an unaerated thoroughly intermixed tank or tank cascade is connected upstream from the activated sludge tank. Return sludge and raw or presettled sewage enter into this tank, and sludge detention times in this tank are from 1 to 5 hours. Detention times in the subsequent activated sludge tank are about 2 to 5 hours. The use of this process is limited to sufficiently highly loaded plants, in which no nitrification (microbial oxidation of ammonia to nitrite and nitrate) occurs. This would result in the nitrite or nitrate with the return sludge entering the unaerated part of the plant and preventing phosphorus redissolution there.
In another process, the so-called "Phoredox" or modified Bardenpho process, as in the A/O process, return sludge and raw or presettled sewage first enters into an unaerated, thoroughly intermixed tank or into a tank cascade. But in the downstream stages, there are provided a nitrification stage and a complete denitrification stage (microbial reduction of nitrite and nitrate to molecular nitrogen, which escapes as gas from sewage), so that no nitrite or nitrate, with the return sludge, can enter the anaerobic phosphorous redissolution stage. Thus, this process is specially tailored for lightly loaded plants with biological nitrogen and phosphorus elimination. In this case, in the first unaerated, thoroughly intermixed tank serving for phosphorus redissolution, sludge detention times on an order of magnitude of 1.5 hours are obtained.
Another process is the so-called UCT (University of Cape Town) process, which is described, for example, in Ekama, G. A., Marais, G. R., Additional biological P elimination in the activated sludge process--Experiences in South Africa, GWF 126, pages 214 to 249 (1985). This process was developed to be able to avoid the high expenditure necessary for a complete denitrification. In this process, there is a deliberate limitation to an upstream denitrification, by means of which, for reasons of principle, a 100% nitrite or nitrate elimination can never be achieved. Since the return sludge thus contains nitrite and nitrate, it is first fed into a denitrification tank. From this tank the sludge is then fed into the upstream, unaerated mixing tank, which is to serve for phosphorus redissolution. There, as in the two first mentioned processes, the contact with raw or presettled sewage takes place with complete intermixing, and the sludge detention times are on an order of magnitude of 1.5 hours.
The three processes described so far are the three main biological processes for phosphorus elimination. Another process, the so-called Biodenipho process, basically does not differ from the processes already described as far as achievement of the phosphorus redissolution is concerned.
In comparison with the processes described so far, there are substantial differences in the so-called Phostrip process, which is described, for example, in Levin, G. V., Topol, G. J., Tarnay, A. G., Operation of Full Scale Biological Phosphorus Removal Plant, JWPCF 47, 577-590 (1975). This process represents a combination of the enhanced biological phosphorus elimination with a chemical precipitation process for phosphorus removal. In this process a part of the return sludge is fed into a settling tank described as "stripper." While the phosphate redissolution starts in this tank on the basis of an anaerobic detention time lasting several hours, at the same time the separation of a phosphate-containing supernatant takes place by a static thickening. This phosphate-containing supernatant water is then chemically precipitated.
Thus, in contrast with the other processes, in the Phostrip process, the objective is the elimination, by chemical sewage partial stream precipitation, of the portion of the phosphorus load which cannot be eliminated from the sewage by means of purely biological effects. The specific features of a precipitation with lime (Ca(OH).sub.2) call for the lime to be used much more economically in the sewage partial stream precipitation than in the precipitation from the total sewage stream.
In principle, the use of the Phostrip process is limited to nonnitrifying, that is, highly loaded activated sludge plants, since nitrite or nitrate can enter into the stripper with the return sludge. But corresponding countermeasures are possible and have already been achieved. The last named process was repeatedly modified, first, with the goal of "acceleration of the phosphorus redissolution in the stripper" (feeding of presettled sewage), but especially with the goal of "considerable conversion of phosphate in the stripper supernatant" (feeding of presettled sewage, purified sewage or chemically treated sewage, recycling of stripper sludge).
The efficiency of the purely biologically operating process for phosphorus elimination (A/O, Phoredox, UCT, Biodenipho) so far has proved to be inadequate in most cases. Because of the process requirements differing from one another, so far the results documented in the literature can hardly be compared with one another, and so far parallel studies are completely lacking. However, it can be stated that an overall phosphorus elimination of over 75% can be achieved only in very rare cases.
The substantially higher attainable elimination rates in the Phostrip process are to be attributed to the chemical partial stream precipitation. Also in this process the biological portion in the total elimination performance is limited to under 75%.
Possibly with the exception of the A/O process, all the processes described so far have the serious disadvantage of being highly complex, involving a high investment cost, and moreover, the operation can be maintained only with especially trained personnel.